Traction Harmonics

7/28/2019 Traction Harmonics

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Delivered by Dr C T TseNov-14, 2011 (Mon)

FJ303, PolyU

IEEE/HKIE

Seminar

Impact of Traction Harmonics

to Power System

412

Power quality issues

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Appears to have no harmonic current limits at 132kV

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Major Abnormalities due to AC traction

1. Voltage fluctuation

2. Voltage dip (Vmin is 17.5kV)

3. Voltage and Current Imbalances

4. Voltage and Current Harmonics

5. Interferences with signalling andcommunication system (to be discussed in EE537).

(6). Low power factor

In order to apply for the economic (bulk) tariff, the traction

operation has to comply with the regulation/limits imposed bythe power utility with respect to items 1, 3, 4 & 6, at the point of

common coupling (PCC), e.g. Fanling 132kV


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AC Traction is the only consumer that contributes all theabove abnormalities.

Remedy

Install booster transformers

sectionalize the railway system

install capacitor compensator/filter at strategic locations

(to be discussed in EE533)

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Traction Harmonics

Adverse Effect of HarmonicsOverheating of conductors

Overheating of electrical equipment

Mechanical oscillation of electrical machine

Telecommunication interference

Inaccurate meter readings

Disturbance to sensitive electronic equipment

False operation of protection equipment

Harmonic Source

AC & DC drives

Standards

Engineering recommendation G5/3, G5/4

IEEE standard 519-1992

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AC Locomotives with Tap Changer Control

Power collected by pantograph and passed to transformer

25kV stepped down and then rectified to acceptable voltage for motor (dc)Current controlled by Tap Changer (instead of conventional resistor)

(DC traction motor has many problems.)8

Single-phase AC powering 3-phase motor

AC Locomotives with PWM Control

Single phase 50Hz AC (after rectification) becomes 3-phase AC

with variable voltage and variable frequency (VVVF), supplying

3-ph motors.


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Harmonic current (Filter tuned at 2.9x50Hz and with 22MVAr compensation)

100.00100.00100.00100.00100.00100.00IF +ITSource IS

42.7940.9037.4729.995.59-22941.7ITTransf.

57.2159.1062.5370.0194.4123041.7IFFilterHarmonic

Current

(%)

1.261.080.900.720.540.3600.18XTTransf.

0.940.750.540.310.03-0.358-1.20XL+XCFilter XF

-0.20-0.23-0.27-0.34-0.46-0.68-1.37XCCapacitor

1.140.980.810.650.490.330.16XLInductor

Reactance

(pu)

76543250Hz

Harmonic number

Controlled 25kV series resonance to absorb more third harmonics (94%)

25kV parallel resonance and I2 is much amplified (where X25XT//XF)20Uncontrolled 132kV series resonance according to X132XT+XF

and external (132kV) I3 magnitude is unknown.

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Characteristics of new drive of SP1900 (IKK) train

Unity power factor

Rich in low harmonics with some high-order harmonics

Passive filter (causing over-compensation and overvoltage at

50Hz) is inappropriate for installation.In a consultancy study of including IKK train in the East Rail

(one IKK with 4 convention MLR), for a scenario of the only

IKK train in powering mode:

- poor and negative power factor = -0.427,

- over-compensation by 3MVAr andover-voltage (V=1.073pu)

Other Problems:

High-order (over 50th) harmonics generated by unity pf drives

Passive filter tuned at, say, n=50.5 must amplify harmonics h


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System modeling at 50Hz

System modeling at h harmonic

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Harmonic current flows at 132kV

Ih (small) from traction is injected to 132kV system via

PCC, and will return via Isys (positive) and IB (negative)

ISYS at a s/s is much amplified if hXSYS 1/hB (parallel resonance)

To meet hXSYS 1/hB, the location of resonance (B), theharmonic order (h), and the time in a day (XSYS) can vary.

Note the approximate equivalent circuit does not include 25kV

(i.e. not related to filter design), and this 132kV resonance may

not be detected in KCR.

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Fortunately, many Rs in the two Z branches and connected loadswill attenuate current amplification in parallel resonance, if any.

Appro. equivalent

circuit at s/s

if Zh0

hXSYShB

Ih

ISYS IB

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Effect of 3rd harmonic current in neutral wire for 3-phase

1=t for fundamental, n=nt for nth harmonicFor the same time span t, n=n1When 3th harmonics completes one cycle, thefundamental goes through only 120

If a system has, say, 40% 3rdharmonic,

let I1=1, IP=(12+0.42)=1.077, IN=30.4=1.2,IN>IP and the neutral wire may be overloaded.

Under balanced load, the neural wire current

IN=IR+IY+IB=0 for fundamental 50HzBut, their 3rdharmonics are in-phase

IR3=IY3=IB3 and IN3=3IR3This also applies to harmonics of 6 th, 9th, .

0 50 100 150 200 250 300 350 400

0 50 100 150 200 250 300 350 400

0 50 100 150 200 250 300 350 400

IR1

IY1

IB1

IR3

IY3

IB3

1=120

Harmonics in 3-ph system:

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Harmonic in Automated People Mover (APM) System for Airport

Whilst I5 is absorbed by 5th

harmonic filter (


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Traditional Concept on Singly tuned Filter

The absorption concept at >ralso applies, but this concepthas overlook that negative reactance X at Vmax since

VC and VL are of opposite sign

Filter design based on s=Vmax/Vcap may lead to capacitor

insulation failure under higher voltage stress.

Vmax=27kVVC

25kV bus

VL

It should be s=Vmax/Vcap+XLCp

With passive filter, the 50Hz capacity compensation will be excess when less

train at powering mode and may lead to over-compensation and over-voltage.

The max. allowable voltage for KCR ac traction drive is 27.5kV.

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(Details to be provided in EE510)

Suppose each cap has a voltage rating of Vcap=4.5kV.

For Vmax=27kV, the number of cap in series

appears to be s=Vmax/Vcap=27/4.5=6


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Harmonics in dc traction system

In 3-ph system, DC supply obtained from full wave rectifier is common.

D1

D6

D2D3

D4

D5

VRY

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The DC output voltage has six sections

in one cycle (i.e. the so-called 6-pulserectifier), and each section is of 360/6=60.

DC ripple can be reduced by more pulse rectifiers

60

360

DC

ripples

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12-pulse Rectifier

If a 3-ph Tx has two sets of secondary windings of star and delta

connections, the secondary line voltages will have an angle

difference of 30.

B

A

C

Primary

a1

b1c1V

bc

c230

Vbc

a2

b2

For 12-pulse rectifier, harmonic current Ih with h=12k1 (i.e. 11,13, 23,25, 35, 37 ) will exist at the Tx primary, and Ih/I1=1/h is simply the

reciprocal of h which is rather small at high h values.

(I1 is the fundamental 50Hz Tx current.)

In 24-pulse rectifier, Ih for h= 11, 13, 35 & 37 are further suppressed.

Two

Secondary

If a rectifier is fed by these two secondary windings, the rectifier output

VDC will be of 360/30=12 pulses, and the DC ripple is smaller than that

of the 6-pulse rectifier.

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Each Tx has zig-zag primary

winding, such that one Tx winding

of7.5phase shift and the other+7.5 (by means of phase shiftchange switch), i.e. an angle

difference of 15

24-pulse rectifier in DC traction system

Two

12-pulse

rectifiers

In MTR, the each 1.5kV source is a pair of Tx rectifier of 12-pulse each.

These Tx are connected to 33kV systems (of both CLP and HEC), in

which they are split to avoid power circulation.

Each Tx has 2 secondary windings (star and delta).

Then VDCwill be of

360/15

=24 pulse.

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Harmonic suppression by 24-pulse rectifier

The two primary current have an angle difference 1=7.5-(-7.5)=15 at 50Hz,andh=7.5h-(-7.5h)=15h at harmonic frequency, given by:

Reduction factor (RF)

RF = cos82.5=0.13, and similarly for h=13, 35 & 37.

Thus, only 23th and 25th harmonics can only be rich in

the 24-pulse rectifier with magnitude Ih/I1=1/h.

0.130.130.990.990.130.13RF

555

(195)

525

(165)

375

(15)

345

(-15)19516515h

373525231311h

In MTR, the high harmonic injection to PCC is very unlikely,but the hazard of harmonic resonance beyond PCCfor all h

(due to B of 33kV cable & 33kV cap) still exists.

82.5=7.5h

-82.5

Ih+

Ih

Ih+ + Ih

Current sum of

2 Tx for h=11

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For h=11, 7.5h=82.5, 82.52=165


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Annex-1

Power Quality equipments in

EE Power systems Lab

Installed at EE in 2003 with full support from ABB (HK)

Except the 2 motors, all the PQ equipments and supply panels

(MINIC/MNS) are freely designed/transported/installed by ABB(HK)

for EE Dept of PolyU for teaching/research purposes.

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MINIC & MNS: Power Supply to PQ and other labs

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Overall view of PQ equipments

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4443


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Active Filter

(in parallel with load)

Third Harmonic Filter THF

(at neutral & in series with load)

THF

Local

3-ph Load

R

YB

NTHF

Local

3-ph Load

R

YB

N

3-phase

Load

XC

XL

Active Filter

N

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Motors & Drives

30kW & 11kW motors

Inverter or

Variable speeddrive, driving

30kW motor

Soft starterdriving 11kW

motor

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CT-2 measures total load

current without filterCT-1 measures total

current with filter

AF injects harmonic current (equal but opposite to local harmonics) until

TOTAL harmonic current ofCT-1 reach specified values

The Active Filter senses the current via the current transformer (CT-1)

On-line computes the measured harmonic current: magnitude and angle

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Effect of Active filter


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Annex-2

Traction Harmonics and Research on Active Filter

A proper filter design has to look after both the capacitivecompensation at 50Hz and the anticipated total harmoniccurrent for all foreseeable scenarios.

A passive filter will absorb harmonics generated by trains,but it will inevitably generate MVAr to be absorbed by trainload.

The shortcoming of trains with unity power factor is theincapability to absorb MVAr, resulting in system over-compensation and over-voltage.

Thus, unity power factor may not be beneficial to a system ifa passive filter has to be installed.

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Problem with trains of unity/leading power factor

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Necessity of Active Filter

With advance of Power Electronics, more rich in high orderand multi order harmonics

Advantages:

Without lower harmonic amplification nor resonance

Programmable capability to handle dynamic range ofharmonics

Immune to external harmonics

Applicable to lagging/unity/leading power factor load

Restrictions of existing Active Filter designHarmonic order below h=50

Voltage below 1 kV

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Proposed Research Proposal of more advanced Active Filter

Higher voltage level (11kV and then 25kV)

Faster dynamic response to combat the

high order harmonics.

Optimal selections and design of the power electronic

converters and the coupling transformer Suitable operational voltage to improve efficiency and

improve response

Can handle both single-phase and three-phase applications

Can handle shallow voltage dip of short durations (say lessthan 200ms) in low power installations.

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Annex-3: Revision on Circuit Theory


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Simple Circuit Analyses

(if XL or XC is large.)

but XL and XC values at resonant freq are finite,and resonant V magnitude is restrictive.

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In general, parallel resonance (with both V and I) is more severe than

series resonance (with V only), due to the possibly large internal current.

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Example of series circuit for 2.950=145Hz Filter

Base values: Sb=26.5MVA, Vb=25kV, Ib=Sb/Vb=1.06kA,

At 50Hz, XL1=0.89pu, XC1=-7.52pu, V1=25kV (i.e. 1pu)

VL1=XL1/(XL1+XC1)V1 = -0.134pu (-3.35kV),

VC1=XC1/(XL1+XC1)V1= 1.134pu (28.35kV)

(subscript 1 stands for 50Hz fundamental)

At 150Hz, XL3=3XL1=2.68pu, XC3=XC1/3=-2.51pu,

and close to resonance

For injection of even a very large I3=10A (i.e. 0.00943pu)

Total VL=(VL12+VL32) = 0.136pu, (3.4kV)

total VC=(VC12

+VC32

) = 1.134pu (28.35kV)

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By proper design of the voltage and current ratings for both L & C,

this 145Hz filter should be effective to absorb 3rdharmonic current.1

VL3=I3XL3= 0.025pu,

VC3=I3XC3=-0.023pu

Adverse effect due to series resonance

is marginal with foreseeable I3

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Only slight increase

from VL1 & VC1

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Biography of Speaker

Dr. C.T. Tse was the Associate Professor in the ElectricalEngineering Department, the Hong Kong PolytechnicUniversity (PolyU).

Before joining the Hong Kong Polytechnic in 1990, Mr. Tsewas the Planning Engineer of System Planning Branch in

CLP. His main duty was to look after power system stabilityand the abnormal loads, such as arc furnace and traction.

During his 20-year service in PolyU, Dr. Tse has engaged in7 consultancy investigations associated traction powersupply (3 with KCR, 2 with MTR, one with KCR/MTR andone with an overseas 1.5kV DC project). One of his researchworks was supported by MTRCL via the PolyU TeachingCompany Scheme in 1996

As the Visiting Associate Professor with the EE Dept after

retirement since September 2010, three of his taught MScsubjects (EE510, EE533 & EE537) related with tractionsystems are continued.

Traction Harmonics

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